For the sake of the complete beginner I'm going to
cover all the parts, even those which may seem obvious.

What is a Repeater?
A duplex repeater, in concept, is not really a complicated
device. It's a 'duplexed' two-way radio set that listens on one frequency,
then re-transmits what it hears on another; and does it simultaneously.
These systems are usually located in places of high elevation (on mountains and
tall buildings) and are equipped with large - efficient antennas, extremely low
loss feedlines, and a transmitter and receiver that is very durable and rated
for continuous duty. The end result? People using a repeater get much greater
range from their radio equipment than would be possible talking simplex (from
radio to radio). This
is how an individual with a portable walkie-talkie (handheld) transceiver can
communicate with people many miles away with good clarity. Repeaters
are used in Commercial (Business) Communications, Emergency Communications (either
by 'hams' or by Federal or Local Government agencies), and even Pleasure
Communications. These machines might have multiple sources of power, including
batteries and/or generators for when commercial power is lost. Repeaters can
be built that are extremely power efficient, and may run exclusively from batteries;
recharged by solar, wind or water power.

What is Simplex?
Simplex is point to point communications without the use of a repeater. Simplex
operation utilizes the same frequency for receive and transmit, like a CB radio.
I.E. Portable to Portable or Mobile to Mobile. The commercial 2-way world calls
Simplex operation 'Talk Around" because you are talking around the repeater, not
through it.

There are such things as Simplex Repeaters. These machines listen
on the frequency for activity, when it recognizes something it will
begin to record that activity for a pre-determined time; usually 1
minute. A slang term for these is a "parrot repeater".
After the activity ceases or the time has expired, the unit will repeat
what it has recorded. This method of communications is somewhat
cumbersome over a conventional repeater; because you are forced to listen to what
you said earlier in time and the channel usage is problematic as you never
know when someone else is recording; however it should not be discounted as these
types of systems can be very beneficial.

What is Duplex?
The simple explanation of full duplex operation is like the telephone,
where both people can talk at the same time. In contrast, a pair of
handhelds operate in half-duplex mode because only one person can talk at a
time. Since the 'repeater' listens and talks at the same time in relaying
your message, it operates in full duplex mode. Here
is another explanation.

How does a Repeater work?
At first glance, a repeater might appear complicated, but if we take
it apart, piece by piece, it's really not really so difficult to understand.
A basic repeater consists of several individual pieces that, when connected,
form a functional system. Here's a simple block diagram of a repeater:

Antenna -
Most repeaters likely use one antenna. The antenna serves both
the transmit and receive RF (Radio Frequency) signals that are going in
to and out of the repeater. It's generally a high performance, durable,
and rather efficient antenna located as high on a tower or structure as
we can get it. Antenna systems of this type can easily cost $500
or more, and that's not including the feedline. On the other hand, when
properly installed and maintaned they can last from 10 to 25 years.

Feedline -
The feedline on most repeaters isn't just a piece of standard coax
cable, it's what's called Hardline. This stuff is more like a pipe with
a center conductor than a cable. It's hard to work with and expensive.
So why do we use it? Performance! The signal loss is much lower in
hardline than in standard cable, so more power gets to the antenna and
weaker signals can be received. A hard rule is that once any percentage
of a received signal is lost that you can't get it back. Remember, the
signal at a repeater site doesn't just travel a few feet to an antenna
like in a mobile rig. It may go hundreds of feet up the tower to the
antenna. Just for fun check out the specs on a roll of coax some
time and see how many dB of loss you'll get from 200 feet of
cable. Hardline also tends to be more durable than standard cable,
which increases reliability and helps us minimize tower climbs to replace
it.

Duplexer -
This device serves a critical role in a repeater. To make a long
story short, the duplexer separates and isolates the incoming signal
from the outgoing and vice versa. Even though the repeaters input and
output frequencies are different, the duplexer is still needed. Why?
Have you ever been in a place where there's lots of RF activity, and
noticed the receive performance of your radio degrades to some degree?
This is called desensitization, or desense, and it's a bad thing on a
repeater. The receiver goes deaf or gets desensitized from the
strong RF signals being radiated in its vicinity and confused about
which signal it should receive. The result is poor receive quality,
or in extreme cases, complete lack of receive capability. Keep
in mind that in this example, the radios are picking up radiated power
from one another and that's enough to cause trouble. Now imagine how much
trouble there will be if you not only have the transmitter and receiver
close together, but connect them to the same antenna! Transmitting only
a few hundred kHz away in frequency would blow away the input to the
receiver if the equipment was simply connected together with a Tee. That's
where the duplexer comes in; it prevents the receiver and transmitter from
'hearing' one another by the isolation it provides. And the more isolation
the better.

A duplexer is a device that is referred to by several different
names like cavities or cans. A duplexer has the shape of tall
canisters and is designed to pass a very narrow range of frequencies
and to reject others. There is some loss to the system because
of the duplexer, however, the advantage of being able to use a single
antenna usually outweighs the drawbacks.

The collection of the antenna, the feedline, the duplexer, and the interconnecting cables is frequently called the "antenna system".

Receiver -
Receives the incoming signal. This receiver is generally a very sensitive
and selective high performance one which helps weaker stations to be heard
better by the repeater. It's also where CTCSS (Continuous Tone Coded
Squelch System) or "PL" decoding takes place. More on this later.

Transmitter -
Most machines have a transmitter composed of two parts: an 'exciter' and a
power amplifier. The exciter created low level RF energy on the proper frequency
and then modulates it with the audio. The power amplifier stages simply
boosts the level so the signal will travel further. Transmitters come in two
types: intermittent duty and continuous duty. One that is rated for
continuous duty is preferred.

The "Station" -
The term "Station" is used to describe a stationary two way radio set;
which includes the transmitter, receiver and sometimes the control
circuitry. One example is the dispatch radio for a fire
department. A 'Repeater Station' is a station designed to
be used as a duplex repeater.

Controller -
This is the brain of the repeater. It handles station identification
(through either CW or voice), activates the transmitter at the
appropriate times, controls the autopatch, and sometimes does many
other things. Some machines also have a DVR (Digital Voice Recorder)
for announcements and messages. The controller is a little
computer that's programmed and optimized to control a repeater. The
various models of controllers have different useful features like
speed-dial for phone patches, a voice clock, facilities to control a
remote base or linking, etc. The controller gives the repeater its
'personality'. Whenever you're using a repeater, you're interacting
with its controller.

What is a Phone Patch or Autopatch? AKA "The Patch"
Many repeaters have a feature that allows you to place a telephone
call from your radio. Phone calls are generally restricted to
the local calling area of the repeater to avoid long distance charges
to the repeater's sponsors. If in doubt, ask if the repeater has
an open patch and how to access it. When using the patch it is
common courtesy to announce your intentions, e.g. " This is N3XZY on
the patch". This may help to prevent anyone from keying up while
you are trying to use the function.

DVR -
A DVR is a Digital Voice Recorder, or in modern terms a "voice mail"
system for the repeater. Usually it's an option that is installed
into the controller.

Repeater Operation -
Operating using a repeater isn't difficult. A good source of info is
the ARRL Repeater Directory. It's an inexpensive book with repeater
listings all over the US. It contains frequency, offset and whether
the repeater is + or - in shift (see "offset" below), whether or
not it requires a PL tone, and other features (like repeater-to-repeater
linking).

What is Offset?
In order to listen and transmit at the same time, repeaters use two
different frequencies. On the 2 meter ham band these frequencies
are 600 kc's (or 600 kilohertz) apart. As a general rule in the
USA, if the output frequency (transmit) of the repeater is below 147 Mhz
then the input frequency (listening) is 600 kilohertz lower. This is referred
to as a negative offset. If the output is 147 Mhz or above then
the input is 600 kilohertz above. This is referred to as a positive
offset. However in any given area the offset rules can be different.

Virtually all ham radios sold today set the offset once you have
chosen the operating frequency. As an example one repeater
output is 145.270 Mhz. The input, or the frequency it listens on is
144.670 Mhz ( 600 kilohertz below). If you have your radio tuned
to 145.270 Mhz with the offset enabled, when you push the PTT switch
(Push-To-Talk) your radio automatically transmits on 144.670 Mhz. When
you release the PTT to listen, the radio reverts back to 145.270 Mhz to
listen on the repeater's output frequency. Note: There are exceptions to
the rule so check local repeater listings.

Why do Repeaters use an Offset?
To use a repeater a user station must use a different transmit
frequency than receive frequency. This is a form of
duplex, or two frequency operation. It is known as half-duplex
as you do not receive and transmit at the same time but normally
use the push-to-talk button on your microphone to switch between
the two.

Most repeater installations use the same antenna for transmit
and receive. Without having an offset of 600 kHz the repeater
would simply hear itself when it was transmitting on the same
frequency it was listening on. Even with the offset, the two
frequencies are close enough that some isolation is required. Again,
this isolation is afforded by the Duplexer.

What is CTCSS or a PL Tone?
PL, an acronym for Private Line, is Motorola's proprietary name for
a radio communications industry standard signaling scheme call the
Continuous Tone Coded Squelch System, or CTCSS. General Electric
uses the name "Channel Guard" or CG for the same system. Other names,
such as Call Guard or Quiet Channel are used by other manufacturers. No
matter what the name, the system is used to prevent a repeater from
responding to unwanted signals or interference. Tone Squelch is an
electronic means of allowing a repeater to respond only to stations
that encode or send the proper tone. Any station may be set up to
transmit this unique low frequency tone that allows the repeater
to operate. If a repeater is "in PL mode" that
means it requires a CTCSS tone to activate the repeater. In days
of old, repeaters that used PL were considered to be closed or
private. This is no longer the case as tone operation has become
more the rule instead of the exception. CTCSS is often referred to
as PL, Channel Guard, and others.

How do you call someone on an Amateur Repeater?
First, listen to make sure that the repeater is not already in use.
When you are satisfied that the repeater is not in use, begin with the
callsign of the station you are trying to contact followed by your
callsign. e.g. " W3ABC this is N3XYZ". If you don't establish
contact with the station you are looking for, wait a minute or two and
repeat your call.

If you are just announcing your presence on the repeater it is
helpful to others that may be listening if you identify the repeater you
are using. e.g. " This is N3XYZ listening on 6-2-5". This allows people
that are listening on radios that scan several repeaters to identify
which repeater you are using.

If the repeater you are using is a busy repeater you may consider
moving to a simplex frequency (transmit and receive on the same frequency),
once you have made contact with the station you were calling. Repeaters are
designed to facilitate communications between stations that normally
wouldn't be able to communicate because of terrain or power limitations. If you
can maintain your conversation without using the repeater, going
"simplex" will leave the repeater free for other stations to use.

Repeater Etiquette
The first and most important rule is LISTEN FIRST. Nothing is
more annoying than someone that "keys up" in the middle of another
conversation without first checking to make sure the repeater is free. If
the repeater is in use, wait for a pause in the conversation and simply
announce your callsign and wait for one of the other stations to
acknowledge your call.

When you are using the repeater leave a couple of seconds between
exchanges to allow other stations to join in or make a quick call. Most
repeaters have a "Courtesy Tone" that will help in determining how long
to pause. The courtesy tone serves two purposes. Repeaters have a time out
function that will shut down the transmitter if the repeater is held on
for a preset length of time (normally three or four minutes). This ensures
that if someone's transmitter is stuck on for any reason, it won't hold the
repeater's transmitter on indefinitely.

When a ham is talking and releases the push-to-talk switch on their
radio, the controller in the repeater detects the loss of carrier and
resets the time-out timer. When the timer is reset, the repeater sends out
the courtesy tone. If you wait until you hear this beep (normally a couple
of seconds), before you respond, you can be sure that you are pausing a
suitable length of time. After you hear the beep, the repeater's
transmitter will stay on for a few more seconds before turning off. This is
referred to as the "tail". The length of the tail will vary from repeater to
repeater but the average is about 2 or 3 seconds. You don't have to wait for the
"tail to drop" before keying up again, but you should make sure that you
hear the courtesy tone before going ahead.

Note: If you don't wait for the beep the time-out timer to may not
reset. Some repeater clubs have a rule that if you time-out the
repeater you get to buy a round of coffee at the next ham club meeting.

What is Doubling?
When two stations try to talk at the same time the signals mix in the
repeater's receiver and results in a buzzing sound or squeal. When you
are involved in a roundtable discussion with several other stations it
is always best to pass off to a specific person rather than leave it up
it the air. e.g. "W3ABC to take it, this is N3XYZ" or "Do you have any
comments Fred?, this is N3XYZ". Failing to do so is an
invitation to chaos and confusion.

It is for this very reason that when groups hold scheduled Nets
(network of hams meeting on air at a predetermined time), they assign a
Net Control station. The Net Controls job is to make sure there is an
orderly exchange and that all stations get a chance to speak. Listen
to a local net and you will get an idea of the format and how the Net Control
juggles the various stations and traffic. It's a job almost anyone can
handle, but as you will discover, some are much better at it than
others. A well run net is both informative and entertaining!

What is a Control Operator?
The Part 97 of the FCC Rules requires all stations in the Amateur
Service that are capable of operating unattended must be monitored for
proper operation while in the unattended mode. This monitoring function
is accomplished by a control operator. The Control Op can be the
licensee of the station or anyone he or she chooses. In many cases,
he or she also ends up being the person that answers questions about the
repeater.

What is White Noise?
White noise is a term used to describe a spectrum of broad band noise
generated in a receiver's detector and sampled to control the
receiver's squelch. When you open the squelch control and hear
the rushing noise from the speaker, this is white noise. The receiver squelch
circuit uses thje presence of that noise to decide that the squelch circuit
should mute the receiver speaker.

I hope this article has explained the Repeater in enough detail
that you understand what it is and how to use it. If there is any
part of this article that seems vague or confusing, please write me and
I'll do my best to explain it better....